It is very expensive to drill bore holes in the earth such as those made in connection with oil and gas wells. Oil and gas bearing formations are typically located thousands of feet below the surface of the earth. Accordingly, thousands of feet of rock must be drilled through in order to reach the producing formations. Additionally, many wells are drilled directionally, wherein the target formations may be spaced laterally thousands of feet from the well's surface location. Thus, in directional drilling, not only must the depth but also the lateral distance of rock must be penetrated.
The cost of drilling a well is primarily time dependent. Accordingly, the faster the desired penetration location, both in terms of depth and lateral location, is achieved, the lower the cost in completing
While many operations are required to drill and complete a well, perhaps the most important is the actual drilling of the bore hole. In order to achieve the optimum time of completion of a well, it is necessary to drill at the optimum rate of penetration and to drill in the minimum practical distance to the target location. Rate of penetration depends on many factors, but a primary factor is weight on bit.
Directional drilling is typically performed using a bent sub mud motor drilling tool that is connected to the surface by a drill string. During sliding drilling, the drill string is not rotated; rather, the drilling fluid circulated through the drill string cause the bit of the mud motor drilling tool to rotate. The direction of drilling is determined by the azimuth or face angle of the drilling bit. Face angle information is measured downhole by a steering tool. Face angle information is typically conveyed from the steering tool to the surface using relatively low bandwidth mud pulse signaling. The driller attempts to maintain the proper face angle by applying torque or drill string angle corrections to the drill string.
Several problems in directional drilling are caused by the fact that a substantial length of the drill string is in frictional contact with and supported by the borehole. Since the drill string is not rotating, it is difficult to overcome the friction. The difficulty in overcoming the friction makes it difficult for the driller to apply sufficient weight to the bit to achieve an optimal rate of penetration. The drill string exhibits stick/slip friction such that when a sufficient amount of weight is applied to overcome the friction, the drill the weight on bit tends to overshoot the optimum magnitude.
Additionally, the reactive torque that would be transmitted from the bit to the surface through drill string, if the hole were straight, is absorbed by the friction between the drill string and the borehole. Thus, during drilling, there is substantially no reactive torque at the surface. Moreover, when the driller applies drill string angle corrections at the surface in an attempt to correct the bit face angle, a substantial amount of the angular change is absorbed by friction without changing the face angle in stick/slip fashion. When enough angular correction is applied to overcome the friction, the face angle may overshoot its target, thereby requiring the driller to apply a reverse angular correction.
It is known that the frictional engagement between the drill string and the borehole can be reduced by rocking the drill string hack and forth between a first angle and a second angle. By rocking the string, the stick/slip friction is reduced, thereby making it easier for the driller to control the weight on bit and make appropriate face angle corrections.